Acidic-wastewater material and method of treating acidic wastewater

ABSTRACT

This invention relates to a method for treating acidic waste water, particularly mine effluent, and to a solid waste water treating material useful for the method. This waste water treating material is obtained by solidifying a mixture of rock wool and an inorganic binder mainly containing at least one kind selected from silicates, hydroxides and oxides of alkaline earth metals and alkali metals and has a porosity of 50% or more. When brought into contact with acidic waste water containing iron ions and sulfate ions, this waste treating material can not only neutralize the waste water but also remove harmful heavy metals such as iron and arsenic. Furthermore, it is easy to dispose the spent waste water treating material.

FIELD OF TECHNOLOGY

[0001] This invention relates to solid waste water treating materialsand a method for treating acidic waste water and, in particular, itrelates to a method for neutralizing acids in mine effluent to removeheavy metals such as iron and arsenic.

BACKGROUND TECHNOLOGY

[0002] Waste water from acidic hot springs in volcanic regions, acidicmine effluent and acidic underground water in regions of volcanic soilcontain sulfuric acid formed by the oxidation of sulfur-containingsubstances and iron sulfide ores. such acidic waste water exerts anadverse influence on concrete structures such as bridges and dams and,still more, acids and heavy metals such as iron and arsenic thereincontained degrade water quality, exterminate fish and shellfishes andcause the so-called “red river” phenomenon when dischargedindiscriminately. For this reason, it is necessary to submit acidicwaste water to a neutralization treatment.

[0003] A method in wide use for this neutralization treatment consistsof adding particles or a slurry of slaked lime to waste water. Thechemical substance used in this method is relatively low in cost and hasan excellent ability to neutralize acidic waste water; however, in thecases where waste water contains sulfate ions and iron ions in largequantities, the iron ions precipitate taking the form of colloidalferric hydroxide as the pH rises and, besides, the sulfate ions reactwith slaked lime to form difficultly soluble gypsum and precipitatetogether with some of the unreacted slaked lime to form a slimy matterthat is high in water content and difficult to dewater. While this ishappening, heavy metals such as arsenic in waste water are adsorbed onthe ferric hydroxide and precipitate together. As this slime is a highlyhydrous slurry that is difficult to dewater and contains harmfulsubstances, its disposal requires installation of a solid-liquidseparator such as an expensive thickener, a sedimentation basin and adevice for dewatering and compacting slime such as a labor-consumingfilter press and construction of a dam for accumulating slime as a finaldisposal device. Thus, the disposal of slime poses problems of increaseddisposal cost and harmful influences on natural environment.

[0004] In order to reduce formation of slime that is highly hydrous anddifficult to dewater, the use of magnesium oxide particles as aneutralizing material has been studied because magnesium oxide formsslime that is easier to dewater and does not form difficultly solublereaction products such as gypsum, but a high cost of the chemical inquestion is a disadvantage.

[0005] Furthermore, in order to reduce cost and improve the dewateringperformance of slime, the use of calcium carbonate particles andlimestone grains has been tried. However, gypsum resulting fromneutralization covers the surface of calcium carbonate or limestone tohinder a further progress of neutralization thereby reducing theefficiency in utilization of the neutralizing material. A neutralizingmaterial based on calcium carbonate produces only a small effect forraising the pH and cannot precipitate ferrous ions in waste water asferrous hydroxide. Therefore, a pretreatment becomes necessary tooxidize ferrous ions to ferric ions by aeration or by iron-oxidizingbacteria.

[0006] Application of inorganic fibers to a treatment of waste water asa filtering material or a material for binding microorganisms isdisclosed in JP6-315681A and elsewhere but nothing is taught of the useof inorganic fibers as a material for neutralizing acidic waste water.

[0007] JP2000-73347A discloses drainage materials composed of inorganicfibers and inorganic hydraulic materials for underdrainage, but they areregarded as substitutes for the husks of rice used up to the present.

DISCLOSURE OF THE INVENTION

[0008] Accordingly, an object of this invention is to neutralize acidicwaste water and remove harmful heavy metals such as iron and arsenic.Another object is to provide a solid waste water treating material whichcan prevent an occurrence of “red water” in rivers, is suited forservice over a long period of time and exhibits a distinct ability toremove heavy metals. A further object is to provide a method fortreating waste water which requires no costly equipment such as aneutralization device, thickener and press nor manpower and can bepracticed practically without need of motive power and a source ofelectricity and without requiring maintenance. A still further object isto provide a method for treating acidic waste water which preventsuseless sulfate ions from entering the spent treating material afterexecution of waste water treatment and facilitates waste disposal byreduction of volume and water content.

[0009] The solid waste water treating material of this invention isobtained by solidifying a mixture of rock wool and an inorganic bindermainly containing at least one kind selected from silicates, hydroxidesand oxides of alkaline earth metals and alkali metals and exhibits aporosity of 50% or more.

[0010] Further, this invention relates to a method for treating wastewater containing iron ions or sulfate ions or both and comprisesbringing the aforementioned solid waste water treating material intocontact with said waste water thereby removing 80% or more of the ironions or neutralizing acidic waste water exhibiting a pH of 5 or less toa pH of 6 to 8.

[0011] The solid waste water treating material of this invention(hereinafter also referred to as waste water treating material) isobtained by solidifying mineral fibers with an inorganic binder.

[0012] Mineral fibers containing silicates of alkaline earth metals oralkali metals are used here. Mineral fibers preferably contain 30-50 wt% of SiO₂, 5-20 wt % of Al₂O₃, 30-50 wt % of MgO and CaO, 0-10 wt % ofNa₂O and K₂ O and 0-10 wt % of others. Rock wool and slag wool areexamples of such mineral fibers and rock wool is preferred because ofits distinct ability to neutralize acidic waste water.

[0013] This neutralizing ability is adequate when 10 g of the materialis added to 1000 ml of an acidic solution containing 2500 mg/l ofsulfate ions and 370 mg/l of Fe⁺² ions and exhibiting a pH of 1.8 andallowed to react with the stirred solution at room temperature for 24hours thereby rendering the pH of the solution to 3 to 6, preferably 4to 5, after the reaction. When the pH of the solution is lower than 3after the reaction, the reactivity in neutralization is low and acidsmay remain in the treated water. On the other hand, when the pH of thesolution is higher than 6 after the reaction, the reactivity inneutralization is too high and the components of rock wool constitutingthe waste water treating material leach into the water being treatedand, as a result, the rock wool loses its property as fiber anddeteriorates in water permeability and dewaterability.

[0014] Rock wool is produced by melting a variety of slags such as blastfurnace slag and electric furnace slag, natural rocks such as basalt anddiabase or a mixture thereof in an electric furnace or cupola andfiberizing the molten mass by a centripetal force or a pressurized gas.Rock wool contains CaO, SiO₂ and Al₂O₃ as main components and furthercontains MgO, Fe₂O₃ and others. A typical composition is 35-45 wt % ofSiO₂, 10-20 wt % of Al₂O₃, 0.1-3 wt % of Fe₂O₃, 4-8 wt % of MgO, 30-40wt % of CaO and 1-4 wt % of MnO. Rock wool can readily be processed intogranular products, exhibits excellent water permeability and waterretention, contains dead air spaces suitable for growth ofmicroorganisms and has a function to neutralize acidic waste water onaccount of its basic chemical composition.

[0015] Rock wool to be used in this invention may be virgin rock wool,waste rock wool containing 50 wt % or more of rock wool or recoveredrock wool.

[0016] Virgin rock wool is available in several shapes such as layeredrock wool and granular rock wool and granular rock wool is preferred.Layered rock wool is processed into granular rock wool by a granulatoror a rotary screen and a material with an average particle diameter inthe range of 1-50 mm, preferably in the range of 5-40 mm, is suitable.Also, use may be made of granules obtained by cutting or crushing amolded rock wool article which is obtained by adding recovered rock wooland a binder followed by molding into a board or the like.

[0017] An inorganic binder useful for solidifying rock wool is mainlycomposed of at least one kind of silicates, hydroxides and oxides ofalkaline earth metals and alkali metals, typically Ca, Mg, Na and K.Preferred inorganic binders are one or two kinds or more of cement,water glass, slaked lime, quicklime, magnesia, slag particles and flyash and they are preferably hydraulic and possess a function toneutralize acids. A hydraulic inorganic binder is allowed to harden inthe presence of water.

[0018] The acidic waste water neutralizing ability of a treatingmaterial is preferably such that 10 g of the material added to 1000 mlof an acidic solution containing 2500 mg/l of sulfate ions and 370 mg/lof Fe⁺² ions and exhibiting a pH of 1.8 is allowed to react with thestirred solution at room temperature for 24 hours thereby rendering thepH of the solution to 6 or more. When the pH of the solution after thereaction is short of 6, both inorganic binder and rock wool leach outsimultaneously during the neutralization reaction and the waste watertreating material loses the property as fiber and deteriorates in waterpermeability and dewaterability.

[0019] Hydraulic inorganic binders of the aforementioned kind includecement (typically portland cement), mixtures of latent hydraulicsubstances such as blast furnace slag particles and alkali materials andslaked lime which reacts with mineral fibers such as rock wool to causesolidification thereof. Other types of cement are available besidesportland cement; for example, blast furnace slag cement, fly ash cement,magnesia cement, alumina cement and lime-mixed cement. Portland cementor blast furnace slag cement is preferable.

[0020] The mix ratio of rock wool and an inorganic binder varies withthe kind of inorganic binder and an inorganic binder is normally used ina proportion of 10-60 wt %, preferably 20-50 wt %, of the sum total ofrock wool and inorganic binder. Excessive use of an inorganic binderreduces porosity and deteriorates water permeability.

[0021] The method for mixing rock wool and an inorganic binder is notrestricted and a known mixer such as ribbon blender can be used for thispurpose. In case an inorganic binder is hydraulic, water is added in arequired amount to cause solidification at the time of mixing or at thesite of use after the mixture is transported there or after the mixtureis applied there. It is further allowable to mix additionally a materialreactive with acids such as limestone particles as occasion demands.

[0022] There is no restriction on the shape of the waste water treatingmaterial of this invention and one of preferred shapes is granular. Tomanufacture a waste water treating material which is molded andsolidified in a granular shape, rock wool, an inorganic binder and waterare mixed in a known mixer such as a ribbon blender and rotarygranulator, molded into granules and solidified. The use of granularwool such as granular rock wool is advantageous in that the granulatingoperation can be omitted. In the case of granules, the average particlediameter is 1-200 mm, preferably 5-50 mm.

[0023] Another of preferred shapes is a sprayed structure. The sprayingtechnique used for applying fireproof coating to buildings can beadopted here. This technique performs mixing of granular rock wool,cement and water and spraying at the same time and a mixture of cementand rock wool, a mixture of cement and water or a mixture of rockwooland water may be prepared in advance. In order to provide a layer of thewaste water treating material of this invention by the sprayingtechnique, granular rock wool as mineral fiber and cement as aninorganic binder are sprayed together with water and allowed to solidifyto an average thickness of 5-300 mm, preferably 10-100 mm.

[0024] Another advantageous method comprises preparing a mixture of rockwool and a hydraulic inorganic binder in advance and solidifying themixture by contact with water to give a solid waste water treatingmaterial.

[0025] A still another advantageous method comprises of filling acontainer with a mixture of rock wool and an inorganic binder andsolidifying the mixture by contact with water to give a solid wastewater treating material. This method becomes more advantageous if thiscontainer is a reactor for effecting the contact with water.

[0026] It is necessary for the waste water treating material of thisinvention to have a porosity of 50% or more regardless of its shape. Theporosity is preferably in the range of 70-98%. Moreover, the bulkspecific gravity of waste water treating materials is set in the rangeof 0.1-1.5, preferably in the range of 0.15-1.0.

[0027] The bulk specific gravity and porosity can be determined by knownmethods. The porosity is determined by weighing a cube of 1 cm³ cut froma waste water treating material when it is dry (A g) and again when itis completely impregnated with water (B g) and computing the differenceB−A.

[0028] Concretely, the porosity is determined in accordance with themethod for determining the porosity in three phase distribution of soil(solid phase, liquid phase, and gas phase).

[0029] With the aid of a commercial soil sampler for a soil three phasemeter (available from Daiki Rika Kogyo Co., Ltd.), a sample is gentlycut out of the waste water treating material in the shape of a cylinderwith a diameter of 50 mm and a height of 51 mm at the application site.In case the thickness of the layer of waste water treating material isshort of 51 mm, a required number of layers are added one over anotheruntil the thickness reaches or exceeds 51 mm and a sample is cut out.The cutting operation is performed in compliance with the directions forhandling the soil sampler.

[0030] The sample cut out of the waste water treating material in thismanner is mounted on a commercial soil three phase meter (actual volumeof soil (solid phase+liquid phase)) and an apparatus for measuring thegas phase according to the Boyle's law (available from Daiki Rika KogyoCo., Ltd.) and the actual volume (solid phase+liquid phase) and theweight (wet) of the sample are determined in accordance with theprocedure for operating the apparatuses.

[0031] To determine the proportion occupied by the liquid phase in theactual volume, the sample is dried sufficiently at 110z,900 and weighedand the weight (dry) was subtracted from the weight (wet) to give awater content.

[0032] Since the internal volume of the sample is 100 ml, the porosity(%) can be calculated from the numerical values obtained above asfollows;

Porosity (%)=100−(measured actual volume)+(water content)

[0033] The bulk specific gravity is calculated by dividing the weight(dry) by the internal volume of the sample or 100 ml.

[0034] When the porosity is too high or the bulk specific gravity is toolow, the amount of waster water treating material per unit volumebecomes insufficient and so becomes the neutralization treatment in somecases. When the porosity is too low or the bulk specific gravity is toohigh, a sufficient contact is not attained between acidic waste waterand the treating material.

[0035] The waste water treating material of this invention is applicableto acidic waste water of any kind and is particularly effective fortreating acidic waste water that contains iron ions or sulfate ions orboth and exhibits a pH of 5 or less, preferably 1 to 4.

[0036] The aforementioned solid waste water treating material is usefulas a waste water treating material in the method for treating acidicwaste water of this invention and the method is particularly effectivefor treating waste water that contains iron ions, preferably Fe⁺² ions,and sulfate ions and exhibits a pH of 5 or less, preferably 1 to 4.

[0037] Although there is no restriction on the kind of waste water to betreated by the method of this invention, mine effluent is treatedadvantageously. Mine effluent is waste water discharged from a mine andcontains both sulfate ions formed by the oxidation of sulfur and ferrousions. Mine effluent oozes out of passageways to form a small stream,small streams unite to form a large stream which flows out of thepassageways and mines or accumulates in a depression to be pumped out.Mine effluent is collected in storage tanks and ponds, treated anddischarged into rivers.

[0038] The waste water treating material is also useful for acidic mineeffluent oozing or flowing out of heaps of waste stones containing ores,outcrops of ores, abandoned mining sites such as open-pit mines and slagheaps of a smelting works.

[0039] Acidic waste water, particularly mine effluent, amenable to atreatment by the treating material of this invention is the one thatshows an 8.3 acidity (the amount of alkali consumed to neutralize to pH8.3) or a 4.8 acidity of 300 mg-CaCO₃/l or more and an iron ionconcentration of 30 ppm or more and it is possible to reduce the 8.3acidity to 200 mg-CaCO₃/l or less, the 4.8 acidity to 100 mg-CaCO₃/l orless and the iron ion concentration to 10 ppm or less by the treatmentperformed in accordance with this invention. That is, the pH rises lessand the iron ion concentration decreases more compared with ordinaryneutralizing materials.

[0040] Acids present in mine effluent and in underground water inregions covered by volcanic mud are mostly sulfuric acid while acids inhot springs in volcanic regions are mostly sulfuric acid andhydrochloric acid. The iron ion concentration in mine effluent isnormally in the range of 50-500 ppm and mine effluent containing ironions in an amount exceeding this range can be dealt with by applying alarger amount of the treating material.

[0041] In the cases where the waste water treating material of thisinvention is provided by spraying in a layered structure, it ispreferable to build a sprayed structure at the sites where mine effluentoozes out or where oozed effluent forms a small stream. In such a case,the waste water treating material is applied by spraying at the site oftreatment where waste water oozes out, for example, at the mouth of amine, heaps of waste stones containing ores, outcrops of ores, abandonedmining sites such as open-pit mines and slag heaps of a smelting worksor the treating material is applied to the whole area covering heaps andabandoned sites. At the aforementioned sites where mine effluent flowsat a low rate, a layer of waste water treating material of even amoderate thickness can maintain a longer contact time. Rainwater thathas passed through the waste water treating material of this inventionshows a pH in the range of 8-12 because of leaching of alkali metals oralkaline earth metals contained in the treating material and thisenhanced alkalinity is effective for reducing the activity ofsulfur-oxidizing bacteria and iron-oxidizing bacteria and retarding theoxidation of sulfides contained in ores and slags thereby giving promiseof reduced generation of acidic water.

[0042] At the site where mine effluent forms a large stream, the wastewater treating material of this invention is advantageously applied byproviding layers filled with granular waste water treating material atsuch sites and passing the mine effluent through the layers. In thiscase, it is preferable to control the thickness of filled layers or theflow rate in such a manner as to obtain a contact time of 30 minutes ormore, preferably 1-5 hours, between the effluent and the waste watertreating material. Moreover, the treated effluent is controlled at a pHof 6 to 8, preferably 6.5 to 7.5.

[0043] In the cases where the waste water treating material of thisinvention is used at the sites where mine effluent is first stored in astorage tank or a pond, a granular waste water treating material isadded as it is to the mine effluent or it is packed in a basket-likecontainer and submerged or suspended in the mine effluent. In the caseswhere the spent waste water treating material is recovered and replacedwith a new material, the use of a container offers an advantage.

[0044] It is further possible to fill a treating tank with a waste watertreating material and pass acidic waste water through the tank. In thiscase, an adequate procedure is to pass acidic waste water from the topthrough the tank filled with a granular waste water treating material,collect the effluent in a receiving trough provided beneath the tank anddischarge as treated water. It is adequate here to control the thicknessof the layers of waste treating material in the range of 100-2000 mm andthe contact time in the range of 0.5-5 hours.

[0045] The use of a combination of two methods or more described abovemay be advantageous and these methods can be applied to acidic wastewater other than mine effluent.

[0046] The temperature at which the treating material is kept in contactwith waste water is satisfactorily room temperature and the contact timeis 30 minutes or more, preferably 60 minutes or more, although it varieswith the amount of filled material, throughput of waste water andconcentration of the substances to be treated in waste water.

[0047] In a method for treating waste water containing iron ions orsulfate ions or both, a preferable procedure is to solidify a mixture ofrock wool and an inorganic binder by contact with water to give a solidwaste water treating material having a porosity of 50% or more and bringthis solid waste water treating material into contact with waste watercontaining iron ions to remove 80% or more of iron. In this case, theconcentration of iron ions in the waste water to be treated ispreferably in the range of 100-250 ppm.

[0048] Moreover, with the use of the solid waste water treating materialof this invention, iron ions precipitate when waste water having a pH of3 or less is neutralized to a pH of 4 to 6 by contact with the treatingmaterial and removal of iron is preferably effected under thiscondition.

[0049] In another method of treating waste water containing iron ions orsulfate ions or both, it is advantageous to solidify a mixture of rockwool and an inorganic binder by contact with water to give a solid wastewater treating material and bring this treating material into contactwith acidic waste water having a pH of 5 or less thereby effectingneutralization to a pH of 6 to 8.

[0050] When the waste water treating material of this invention isbrought into contact with acidic waste water, the alkaline earth metalsand alkali metals in the treating material react with acids and silicicacid remains as amorphous silica. Sulfate ions partly react with calciumin the waste water treating material to produce gypsum, but the amountof gypsum thus produced is small because of the presence of otheralkaline earth metals and alkali metals. In consequence, sulfate ionsmostly form harmless water-soluble sulfates and are discharged. Ironions in mine effluent are mostly ferrous ions and, when brought intocontact with the waste water treating material of this invention, theirreaction with the material progresses slowly and in the meantime ferrousions are oxidized by dissolved oxygen and the like to ferric ions andprecipitate as ferric hydroxide. Mine effluent sometimes contains heavymetals such as arsenic and cadmium and most of these metals can beprecipitated and removed by contact with the waste water treatingmaterial of this invention.

[0051] When the waste water treating material of this invention is used,alkaline earth metals and alkali metals decrease and ferric ions formedin a large amount in the reaction precipitate as ferric hydroxide and itis desirable to replace or add the treating material immediately beforethe pH of the treated water or an indicator of the ability as an acidicwaste water treating material drops below the value specified for thetreating site or immediately before the iron ion concentration in thetreated water reaches 10 mg/l.

[0052] Replacement or addition of the treating material is preferablyperformed as follows. When the treating material stops giving thespecified performance in waste water treatment, the treating material isrenewed as follows; rock wool and an inorganic binder are sprayed againwith or without removal of the spent material, a mixture of rock wooland an inorganic binder is added or the spent mixture is replaced with anewly prepared mixture or the container is refilled with a freshlyprepared mixture. In this case, it is preferable to follow the renewalprocedure while adding water to solidify the waste water treatingmaterial and the addition of water may be made either after orsimultaneously with mixing of rock wool and the inorganic binder. Incase the container is to be filled with the mixture, it is advantageousto add water to solidify the mixture of rock wool and inorganic binderimmediately after filling.

[0053] The spent waste water treating material contains the unreactedsilicates, reaction residues mainly containing silica, and reactionproducts comprising a large mount of iron compounds and a small amountof gypsum. Therefore, the spent waste water treating material can beused as an iron-containing soil conditioner and the like and is easilydisposed unless the material in question contains harmful componentssuch as arsenic. It is preferable for the solid waste water treatingmaterial of this invention to contain 50 wt % or more of amorphoussilica in the residual treating material after use in waste watertreatment.

[0054] The rock wool-containing waste water treating material of thisinvention can prevent the treated water from becoming excessivelyalkaline and does not require readjustment of the pH by acid. Moreover,silicate gels formed from rock wool coprecipitate with iron colloids todirectly replace rock wool and form a solid fibrous aggregate and, as aresult, slime which is hard to dewater does not form and at the sametime precipitation of iron contained in waste water is accelerated.Furthermore, since rock wool contains other readily leachable cations,the neutralization reaction is hindered very little by gypsum. Stillfurther, the dewatering performance does not deteriorate very much ifthe treating material is rendered granular for higher waterpermeability. When the products in the spent treating material containlittle harmful heavy metals, the spent material can be utilized as asoil conditioner and the like.

PREFERRED EMBODIMENTS OF THE INVENTION EXAMPLE 1

[0055] Granular rock wool (granular S-FIBER, average particle diameter30 mm, available from Nippon Steel Chemical Rockwool Co., Ltd.) was usedas rock wool.

[0056] To begin with, the test for leaching of rock wool was performed.Rock wool was pulverized finely in a mortar, 1 g of the pulverized rockwool was immersed in 150 ml each of pure water, 2% aqueous citric acid,0.25 N hydrochloric acid and 0.5 N hydrochloric acid and the amounts ofleached alkaline earth metals, alkali metals, silica and alumina weredetermined (expressed in ppm of the components leached from 1 g of rockwool). The results are shown in Table 1. The analysis was carried out inaccordance with the method for analysis of fertilizers.

[0057] It is seen from Table 1 that rock wool reacts not only withhydrochloric acid but also with a weak acid such as citric acid.

[0058] After this, a granular acidic waste water treating material(average particle diameter 20 mm, porosity 95%) obtained by solidifying100 g of a mixture of 60 wt % of rock wool and 40 wt % of blast furnaceslag cement with 100 g of water was pulverized finely in a mortar, thepulverized treating material was added to 1000 ml of acidic effluentthat was sampled at a mine of iron sulfide ores, contained 1300 mg/l ofsulfate ions and 135 mg/l of total iron ions and exhibited a pH of 2.8,batch tests were carried out to determine the changes in pH with timeand the results are shown in Table 2. Removal of total iron ionsdetermined 1 hour after the addition was 99.9%. All the tests werecarried out at room temperature with stirring.

[0059] These results indicate that rock wool by itself cannot removeiron ions, but rock wool converted into a waste water treating materialin accordance with this invention can remove iron ions as the iron ionspenetrate into the material and participate in the reaction.

EXAMPLE 2

[0060] A pulverized molded rock wool article (rock wool for plantcultivation, available from Grodan Co., average particle diameter 50 mm,porosity 92%) was used as rock wool and it was finely pulverized in amortar and tested for leaching as in Example 1 to give the results shownin Table 1.

[0061] After this, the aforementioned rock wool was substituted for therock wool in Example 1 and a mixture of the aforementioned rock wool andblast furnace slag cement was solidified to form a waste water treatingmaterial (average particle diameter 50 mm, porosity 95%), pulverizedfinely in a mortar as in Example 1 and tested for the changes in pH togive the results shown in Table 2. Removal of total iron ions was 99.2%when determined 1 hour after the addition. TABLE 1 Na K Ca Mg Si AlExample 1 Pure water 11 15 230 17 27 4 0.2% Citric acid 720 2000 19000024000 69000 31000 0.25 N HCl 730 1900 170000 24000 43000 30000 0.5 N HCl730 2000 200000 24000 59000 31000 Example 2 Pure water 16 3 22 17 16 100.2% Citric acid 17000 26000 76000 35000 120000 54000 0.25 N HCl 1700025000 75000 33000 110000 54000 0.5 N HCl 21000 29000 90000 39000 12000064000

EXAMPLE 3

[0062] The same rock wool as used in Example 1 was mixed with portlandcement in a ribbon blender in a weight ratio of 60 of the former to 40of the latter, the mixture was sprayed with water weighing the same asthe mixture, and left standing overnight to give a granular solid (awaste water treating material) exhibiting an average particle diameterof 20 mm and a bulk specific gravity of 0.196.

[0063] This waste water treating material (100 g) was added to 1000 mlof the same waste water as used in Example 1 and batch tests werecarried out to determine the changes in pH to give the results shown inTable 2. Removal of total iron ions determined one hour after theaddition was 98.0%. TABLE 2 Time elapsed Example 1 Example 2 Example 3 46.2 6.1 6.4 8 6.3 6.2 6.9 16 6.5 6.6 6.9 24 6.6 6.9 6.9 72 6.9 7.2 7.1

EXAMPLE 4

[0064] The same rock wool as used in Example 1 was mixed with portlandcement in a ribbon blender in a weight ratio of 64 of the former to 36of the latter to give an unsolidified waste water treating materialexhibiting an average particle diameter of 20 mm and a bulk specificgravity of 0.17.

[0065] Then, a container measuring 90 cm in height, 120 cm in length and16 cm in width provided with a plastic net at the bottom was filled with20 kg of the unsolidified waste water treating material to give a 60cm-thick mass with a porosity of 92% and a bulk specific gravity of 0.20and this mass was solidified by adding water of the same weight as themass to give a solid waste water treating material.

[0066] Upon completion of solidification, 50 m³ of acidic mine effluenthaving the water quality shown in Table 3 was passed from the top of thecontainer at a rate of 14.5 L/hr. Addition of the neutralizing materialper 1 m³ of the mine effluent was 0.4 kg/m³ during this treatment.

[0067] The treated water flowing out of the bottom of the containershowed iron removal of 99.9% and arsenic removal of 94.2%. The ironcontent in the spent waste water treating material was 53% and 14% ofthe neutralizing material remained. The water quality of the treatedwater is shown in Table 3.

[0068] The water permeability of the waste water treating material was1.0×10⁻² cm/s initially and 0.6×10⁻² cm/s after passage of 50 t of mineeffluent. The volume of the waste water treating material was 88% of theinitial volume after passage of 50 t of the mine effluent. The watercontent in the waste water treating material after passage of 50 t ofthe mine effluent was 77.2% on the average when determined 30 minutesafter passage of the mine effluent was stopped and the bulk specificgravity after drying at 110° C. was 184 kg/m³. TABLE 3 Water T—Fe As SO₄²⁻ 8.3 acidity 4.8 acidity quality pH mg/l mg/l mg/l mg-CaCO₃/lmg-CaCO₃/l Acidic mine 2.8 90.3 0.12 956 830 768 effluent Treated 4.10.05 0.007 960 64 1 water

COMPARATIVE EXAMPLE 1

[0069] Reagent-grade calcium hydroxide in 325-mesh particles was addedto the same acidic mine effluent as used in Example 4 and the amount ofcalcium hydroxide required to neutralize the mine effluent to the same8.3 acidity as in the case of the treated water in Example 4 wasdetermined. It was 0.58 kg/m³ per 1 m³ of the mine effluent. To 1000 mlof this mine effluent was added 580 mg of slaked lime as a neutralizingmaterial, the mixture was stirred at room temperature for 24 hours andthe entire lot was filtered through a vacuum filter (a suction funnel,inside diameter 70 mm, No. 5C filter paper). The time required forfiltration was 342 seconds. The water permeability of the filteredmaterial was 4×⁻⁶ cm/s.

[0070] COMPARATIVE EXAMPLE 2

[0071] The amount of sodium hydroxide to neutralize the same acidic mineeffluent as used in Example 4 to the same 8.3 acidity as in the treatedwater obtained in Example 4 was determined by adding an aqueous sodiumhydroxide solution (1 mol/l) to the acidic mine effluent according tothe method for analysis of water quality specified by JIS and thisamount was used to calculate the amount of calcium carbonate required toneutralize the acidic mine effluent to 8.3 acidity. It was 0.77 kg/m³per 1 m³ of the mine effluent. To 1000 ml of this mine effluent wasadded reagent-grade calcium carbonate in 325-mesh particles, the mixturewas stirred at room temperature for 24 hours and checked for its pH,which was 6.9. The entire lot was then filtered through a vacuum filter(a suction funnel, inside diameter 70 mm, No. 5C filter paper). The timerequired for filtration was 93 seconds. The water permeability of thefiltered material was 1×10⁻⁵ cm/s.

COMPARATIVE EXAMPLE 3

[0072] The same acidic mine effluent as used in Example 4 was introducedat a rate of 0.05 m³/min to an oxidation tank with an internal volume of1 m³, the same reagent-grade calcium carbonate in particles as used inComparative Example 2 was added at a rate of 0.20 kg/m³ per 1 m³ of themine effluent to keep the pH inside the oxidation tank at 3 to 4 andiron-oxidizing bacteria and air were blown into this reaction systemthereby oxidizing ferrous ions to ferric ions. The reaction mixture wasthen introduced to a neutralization tank with an internal volume of 1m³, the same reagent-grade calcium carbonate in particles as used inComparative Example 2 was added at a rate of 0.57 kg/m³ per 1 m³ of themine effluent, the resulting mixture was stirred to precipitate ferricions as ferric hydroxide, and the effluent containing this reactionproduct was settled in a thickener with a diameter of 2 m and aninternal volume of 5 m³ to effect separation into the treated water andthe reaction product. The reaction product was returned to theneutralization tank at a rate of 0.02 m³/min to raise the utilizationefficiency of the neutralizing material and the dewaterability of thereaction product. The slurry obtained from the thickener showed a watercontent of 99% at this time and the slurry dewatered under a pressure of11 kg/cm² by a filter press with a filtering area of 0.25 m² showed awater content of 76%.

[0073] Removal of iron from the original mine effluent was 99.9% andthat of arsenic was 99.8%, the iron content in the reaction productafter use was 38% and the rate of survival of calcium carbonate was 45%.The concentration of sulfate ions at that time was reduced to 74% ofthat of the original mine effluent. However, compared with Example 4, alarger amount of the neutralizing material was used per 1 m³, the ironcontent in the reaction product was lower and the rate of survival ofunreacted calcium carbonate was higher than those in the examples, andmore unreacted calcium carbonate remained; this means that the treatingmaterial was utilized less efficiently and the reaction products took insulfate ions that needed not to be removed thereby gaining weight andincreasing the work load at the final disposal site. Moreover, theoperation requires extra steps of oxidation by air blowing anddewatering under pressure of the reaction product by a filter press andthis was a shortcoming in respect to labor saving and capital cost.

[0074] Industrial Applicability

[0075] The method for treating acidic waste water by the use of acidicwaste water treating material of this invention effects not onlyneutralization but also removal of harmful heavy metals such as iron andarsenic. The waste water treating material of this invention reacts withacidic waste water thereby causing more to form soluble compounds andless to remain behind, the water permeability is maintained after useand volume reduction becomes possible and a dewatering apparatus such asa filter press becomes unnecessary.

1. (As amended) A solid waste water treating material for treatingacidic waste water containing iron which is obtained by solidifying amixture of granular rock wool and an inorganic binder mainly containingat least one kind selected from silicates, hydroxides and oxides ofalkaline earth metals and alkali metals and has a porosity of 70-98% anda bulk specific gravity of 0.11.0.
 2. A solid waste treating material asdescribed in claim 1 wherein the inorganic binder is at least one kindselected from cement, slaked lime and quicklime.
 3. A solid wastetreating material as described in claim 1 wherein the content ofamorphous silica in the residual waste water treating material after usein waste treatment is 50wt% or more.
 4. (As amended) A method fortreating acidic waste water containing iron ions or iron ions andsulfate ions which comprises solidifying a mixture of granular rock wooland an inorganic binder mainly containing at least one kind selectedfrom silicates, hydroxides and oxides of alkaline earth metals andalkali metals to form solid waste water treatomg material having aporosity of 7098% and a bulk specific gravity of 0.1-1.0 and bringingsaid solid waste water treating material into contact with waste watercontaining iron ions thereby removing 80% or more of the iron ions.
 5. Amethod for treating waste water as described in claim 4 wherein theconcentration of iron ions in 100-250 ppm.
 6. A method for treatingwaste water as described in claim 5 wherein removal of iron is effectedunder such neutralizing conditions as to change the pH of waste waterfrom 3 or less before treatment to 4 to 6 after treatment.
 7. (Asamended) A method for treating acidic waste water containing sulfateions or sulfate ions and iron ions which comprises solidifying a mixtureof granular rock wool and an inorganic binder mainly containing at leastone kind selected from silicates, hydroxides and oxides of alkalineearth metals and alkali metals to form a solid waste water treatingmaterial having a porosity of 70-98 % and a bulk specific gravity of0.1-1.0 bringing said solid waste water treating material into contactwith acidic waste water having a pH of 5 or less thereby effectingneutralization to a pH of 6 to
 8. 8. (As amended) A method of applying awaste water treating material for treating acidic waste water containingiron which comprises applying granular rock wool and an inorganic bindermainly containing at least one kind selected from silicates, hydroxidesand oxides of alkaline earth metals and alkali metals by one ofspraying, mixing and container filling to provide a waste water treatingmaterial having a porosity of 70-98% and a bulk specific gravity of0.1-1.0.
 9. (As amended) In a method for applying a waste water treatingmaterial for treating acidic waste water containing iron, a method forrenewing a waste water treating material which comprises applyinggranular rock wool and an inorganic binder mainly containing at leastone kind selected from silicate, hydroxides and oxides of alkaline earthmetals and alkali metals by one of spraying, mixing and containerfilling to provide a waste water treating material having a porosity of70-98% and a bulk specific gravity of 0.1-1.0 and , upon execution ofwaste water treatment, again applying granular rock wool and aninorganic binder by one of spraying, mixing and container filling withor without removing the spent waste water treating material.